Abstract

A 1980 field survey of earthquake activity near Rocky Mountain House, Alberta, using one digital and six conventional seismographs, recorded 146 microearthquakes (M ≤ 3.4) in 23 days and located 67 of them. The located events, and probably the unlocated events as well, all occurred in a small zone approximately 4 km north–south by 4 km east–west by 1 km thick, centred near 52°12.5′N, 115°15′W at a depth of 4.0 km, with an uncertainty of ±2 km on each of the location coordinates. The activity occurred below and (or) in the Strachan D-3A sour gas reservoir, which is a Devonian-aged limestone reef complex in a section of nearly flat-lying, unfaulted sediments. The earthquakes had a composite thrust-faulting mechanism with generally north-trending, intermediate-dipping planes in the presence of a regional deviatoric stress field that was horizontally compressive and oriented approximately east–west. The earthquake activity may be related to the extraction of the natural gas and related fluids from the reservoir, but the exact relationship cannot be documented at this time.The earthquake activity in the region increased dramatically after the production of gas from the reservoir began in the early 1970's. The tight cluster of activity found by the field survey suggests that all the activity in the region occurred in the same small zone but that earlier events may have been mislocated. Earthquake activity in western Alberta generally is not induced and occurs within a regional seismicity belt spatially continuous with the Intermountain Seismic Belt of the northern United States, but the earthquake activity in Canada seems to be tectonically distinct from that in the U.S.A., being characterized by thrust faulting and horizontally compressive stress.

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